Coalbed gas is a self-generating and self-preserving unconventional natural gas in coalbeds. There are 74 countries in the world that are endowed with coalbed gas resources, while China boasts coalbed gas reserve up to 36.8 trillion cubic meters, ranking No. 3 in the world; China has exploitable coalbed gas resources of about 10 trillion cubic meters. Nowadays, values of such unconventional resource has been recognized; the exploration and utilization of coalbed gas resources have been gradually progressing all over the world.
In the extraction process of coalbed gas, a large amount of coalbed water contained in coalbeds may cause excessively high pressure at the bottom of the coalbed gas well, so that the coalbed gas cannot flow into the well shaft. Therefore, it is required to discharge the coalbed water so as to reduce the pressure at the bottom of the coalbed gas well. In this way, the coalbed gas is able to continuously flow into the well shaft due to a pressure difference generated thereby. In addition, the production characteristics of coalbed gas requires that the coalbed water be stably drained with a reasonable drainage and extraction strength. Due to various restrictions such as topographic conditions, investment scale and national land policies, the drilling mode of multi-well cluster (multiple wells drilled in a well site) is increasingly adopted. As determined by this specific drilling mode, the vast majority of coalbed gas wells have inclined shafts. Combined with a shallow burial depth of coalbed that dictates a small hole curvature radius of the coalbed gas well, the following problems are resulted in the drainage and extraction process of coalbed water.
Firstly, the coal gas well is greatly sloped and having a small hole curvature radius. Even the existing vertical wells have such problems as serious hole deviation and high rate of overall angle change. The commonly used sucker-rod pumps (e.g., tubing pumps and screw pumps) thus suffer from serous abrasion of rods and tubes, resulting in high consumption of rod and tube materials as well as frequent workover operations.
Secondly, the output coalbed water contains coal dust and fracturing sand (this is because all the coal gas wells are put to production after fracturing). This leads to frequent faults (such as corrosion of rods, pipes and pumps, pump blocking and stuck pumps) in the existing coalbed water lifting devices (e.g., tubing pumps, screw pumps and electrical submersible pumps), which cause frequent workover operations.
Thirdly, most of the coal gas wells have a water yield less than the minimal discharge capacity requirement of electrical submersible pumps, and therefore do not comply with the well selection criteria of electrical submersible pumps.
Fourthly, although the operating conditions of sand discharge and oil extraction methods are relatively similar to the technical requirements for coalbed gas extraction through drainage of water and coal dust, the methods have yet to be applied in the field of coalbed gas extraction through water and coal dust drainage. Necessary modifications and improvements are to be made to the technical structure of the well shaft as well as to the methods themselves before they can be applied to coalbed gas extraction through water and coal dust drainage.
To sum up, the conventional coalbed water lifting technology employed nowadays would cause frequent workover operations on coalbed gas wells, significantly increasing the production cost of coalbed gas extraction. In addition, the frequent workover operations on coal gas wells may most easily cause damage to the reservoir bed and directly affect the extraction performance of coalbed gas.